The present invention relates to a pneumatic tire for an automobile constituting a tread portion thereof by a two-layered structure of a cap tread layer and an under tread layer and its fabrication method, further particularly to a pneumatic tire for an automobile simultaneously achieving a reduction in rolling resistance and promotion of driving stability and its fabrication method.
Conventionally, in a pneumatic tire for an automobile, a tread portion is constituted by a two-layered structure of a cap tread layer disposed on an outer side in a tire diameter direction and an under tread layer disposed on an inner side in the tire diameter direction. As the under tread layer, with an object of promoting driving stability, a rubber layer having high hardness is used and with an object of reducing rolling resistance, a rubber layer having low heat generation performance is used. However, the under tread layer is provided with a function of protecting belt layers against oil migration by the cap tread layer (oil barrier function), an amount of blending a softening agent is comparatively small and accordingly, when a rubber layer having high hardness is used, it is inevitable that the under tread layer is necessarily provided with high heat generation performance.
Hence, in Japanese Patent Laid-Open No. 246212/1993, it is proposed that a rubber layer having high hardness of JIS-A hardness 70 through 80 is arranged at a center portion of an under tread layer with a width of 30 through 50% of the grounding width and a rubber layer having comparatively low hardness of JIS-A hardness 50 through 60 is arranged at a shoulder portion thereof to thereby reduce rolling resistance and at the same time to promote driving stability. However, there poses a problem in which according to the above-described method, the hardnesses of the rubber layers constituting the center portion and the shoulder portion are limited to narrow ranges and therefore, the tire function which can be set is restricted.
It is an object of the invention to provide a pneumatic tire for an automobile in which a tread portion is constituted by two-layered structure of a cap tread layer and an under tread layer and in achieving a reduction in rolling resistance and promotion of driving stability, conditions of setting thereof are alleviated.
In order to achieve the above-described object, according to an aspect of the invention, there is provided a pneumatic tire for an automobile in which two layers of belt layers including a first belt layer on a side of a carcass layer and a second belt layer on an outer peripheral side thereof are arranged on an outer peripheral side of the carcass layer in a tread portion and the tread portion comprises a two-layered structure of a cap tread layer and an under tread layer wherein the under tread layer is comprising a center region defined by a range of 30 through 70% of a width of the first tread layer centering on a tire equator and a range of 0.5 through 2.0 mm in a tire diameter direction from the second belt layer and a shoulder region defined by a region extended from both end portions of the first belt layer continuously from the center region to an outer side by 30 mm at a maximum on one side and a range of 1.0 through 5.0 mm as a thickness thereof, the center region and the shoulder region comprising rubber composition members different from each other, wherein the center region is arranged with a rubber layer having a blending system in which JIS-A hardness falls in a range of 65 through 80 and an amount of blending a softening agent relative to rubber 100 weight parts is smaller than a blending amount thereof of the cap tread layer by 10 weight parts or higher, the shoulder region is arranged with a rubber layer having a blending system in which the JIS-A hardness falls in a range of 50 through 70 and the amount of blending the softening agent relative to the rubber 100 weight parts is smaller than the blending amount of the cap tread layer by 10 weight parts or higher and a ratio (tan xcex42/tan xcex41) of loss tangent (tan xcex42) of the rubber layer of the shoulder region to loss tangent (tan xcex41) of the rubber layer of the center region is ⅔ or lower.
In this way, by properly arranging the rubber layer having high hardness to the center region and the rubber layer having low heat generation performance to the shoulder region as the under tread layers, a reduction in the rolling resistance and promotion of driving stability can simultaneously be achieved. Further, usable rubber composition members can be selected from a range wider than that in the conventional case and accordingly, desired tire function can be set.
According to the invention, the loss tangent (tan xcex4) is measured by using a spectrometer for measuring dynamic viscoelasticity under conditions of temperature of 60xc2x0 C., initial strain of 10%, dynamic strain of xc2x12% and frequency of 20 Hz.
In order to realize the above-described tread structure, it is conceivable to integrally extrude tread rubber distributed with a number of kinds of rubber composition members in a tire sectional direction by using an extruder. However, in order to extrude tread rubber all together, it is necessary to mount screws for the kinds of the rubber composition members and auxiliary extruding dies for approximating the tread structure to a shape as aimed at the extruder other than tread dies, it is extremely difficult to provide a desired sectional structure while controlling these, the auxiliary extruding dies for each product needs to prepare similar to the tread dies and accordingly, an increase in equipment investment results.
Further, the tread rubber comprising the extruded product fabricated as described above is provided with a drawback in which a bottom face thereof is liable not to be flat owing to a difference in shrinkage rates in cooling of the respective rubber composition members and an air storage is liable to cause in molding raw tire. It is conceivable as a countermeasure thereagainst to immediately supply the tread rubber to a molding cycle of raw tire without providing the cooling step by retarding the extruding speed to be synchronized with the molding cycle of raw tire. However, this is not regarded as a method preferable in view of quality and efficiency in consideration of a delay of cycle when mechanical failure is caused or an amount of producing failed extruded products before and after setup change.
Hence, according to another aspect of the invention, there is provided a method of fabricating a pneumatic tire for an automobile, the method comprising the steps of wrapping belt layers on an outer periphery of a forming drum and wrapping a strip member comprising an unvulcanized rubber composition member having a width of 5 through 30 mm and a thickness of 0.5 through 3.0 mm on an outer peripheral side of the belt layers in a tire circumferential direction while overlapping at least a portion thereof at every turn, wherein a kind of the unvulcanized rubber composition constituting the strip member is made to differ in accordance with a position of a tire meridian section to thereby form a tread portion having a multiple composition structure.
In this way, in forming the tread portion having the multiple composition structure, the strip member is wrapped in the tire circumferential direction while overlapping at least a portion thereof at every turn and the kind of the unvulcanized rubber composition member constituting the strip member is made to differ in accordance with the position of the tire meridian section to thereby efficiently fabricate the pneumatic tire for an automobile having the tread portion in the multiple composition structure excellent in homogeneous performance.